1. Field of the Invention
The present invention generally relates to a multi-level QAM (quadrature amplitude modulation) system for transferring a digital signal by utilizing multi-level quadrature amplitude modulation. More specifically, the present invention is directed to a QAM communication system capable of increasing signal transmission reliability by employing a transparent error correcting method.
2. Description of the Related Art
In a multi-level quadrature amplitude modulation (QAM) communication system in which multi-bit data such as 4 bit data and 8 bit data are transferred with reference to one signal point on a phase plane coordinate including 2.sup.n ("n" being the data bit number) signal points and original data are reproduced based upon the relationship between the amplitude and phase, utilization efficiency for a frequency becomes high so that this QAM communication system has been widely utilized in digital microwave communications and digital mobile communications.
As previously stated, the signal transmission of the multi-level QAM communication system is carried out by employing the QAM signals produced by synthesizing two orthogonal I-channel and Q-channel signals corresponding to each m-level amplitude-modulated signal. Each of these multi-level QAM signals owns m.sup.2 (=2.sup.n) pieces of signal points. For instance, if "m" is selected to be 16 (n=8), this multi-level QAM signal is equal to 256 pieces of QAM signals having 256 signal points.
In a QAM type receiving system employing synchronous demodulation, a carrier wave is first reproduced from this multi-level QAM signal, and then demodulated by utilizing 2 orthogonal-reproduced carrier waves having different phases with each other at 90.degree. (degrees), and thereafter "n" pieces of digital signal series are obtained in total by way of the multi-level identification. In general, there is a drawback in this QAM receiving system in that the phases of the reproduced carrier waves derived from the carrier wave reproducing circuit have a so-called "phase ambiguity", i.e., the phase becomes any of 0.degree., 90.degree., 180.degree., and 270.degree.. Generally speaking, since a transmission signal series cannot be correctly reproduced if phase ambiguity exists, it is required to employ same means for eliminating the adverse influences caused by this phase ambiguity. To this end, there are some solutions to resolve such a phase ambiguity. That is, for instance, a known signal series is periodically transmitted, whereas the phases of the reproduced carrier waves are discriminated based upon the relationship between this known signal series and the signal which has been demodulated and judged by the reproduced carrier waves having the phase ambiguity at the signal reception side. Otherwise, a transmission information signal is differential-encoded so as to be transmitted, which does not directly correspond to the transmission phase, but corresponds to a relative phase difference of a continuous transmitting symbol. At a signal reception end, when this differential-encoded signal is differential-encoded after being demodulated by the reproduced carrier waves, the phase ambiguity in the reproduced carrier waves can be resolved. In general, since a 1 bit error is expanded to a continuous 2-bit error, the differential encoding/decoding method has the advantage that a circuit arrangement thereof is simple, although the bit error rate in the received signal series is increased as compared with that of the first-mentioned solution method for judging the absolute phase. Moreover, to suppress an increase of a bit error rate caused by a differential coding method, there is another method in which signal point mapping of a QAM signal is quadrant symmetry mapping. In accordance with the last-mentioned method, since the judgement concerning the upper 2 bits of the input digital signals which is determined by the orthogonal axes (i.e., I-axis and Q-axis) on the phase plane is adversely influenced by the phase ambiguity, the differential coding operation is required. However, the judgement concerning other bits thereof which is determined by the respective amplitude levels of the I-axis and Q-axis, is not adversely influenced by the phase ambiguity, so that no differential coding operation is required.
Although the QAM modulation method has the advantage of higher frequency utilization, there is a drawback in that when the number of the bits transmitted with 1 symbol, namely the value of "n", is increased, the bit error rate is deteriorated even when the transmission power per 1 bit is selected to be equal. Under such a circumstance, it is required to improve the bit error rate in the multi-level QAM communication system by employing an error correcting method. On the other hand, a QAM modulation system is originally employed so as to increase the frequency utilization efficiency, and accordingly, there is a severe restriction in the available frequency band in systems which employ the QAM modulation method, such as a digital microwave radio communication system. As a consequence, it is expected to utilize a higher coding rate having a less redundant bit to be added to the input digital signal in the error correcting method.
Furthermore, various limitations are provided in applying the error correcting method to the QAM communication system with employment of the above-described differential coding system. First, when the error correcting encoder and decoder are provided outside the differential encoding/decoding processors, since the 1 bit error occurring on the signal transmission channel is expanded to the 2-bit error due to the differential decoding process, the loads required for the error correcting encoder and decoder become large. In other words, error correction codes having greater correction capability are required so as to achieve the same reliability as that of the other case where the error correcting encoder and decoder are provided inside the differential encoding/decoding processors. As a result, since the redundant bit number to be added to the input digital signal is increased, there are problems in that the resultant utilization efficiency of frequency is lowered and the circuit arrangement of the error correcting decoder becomes extensive.
It should be understood that the expression "outside" and "inside" described above are defined as follows. That is, for instance, the error correcting encoder and decoder are positioned outside the differential encoding/decoding circuits in a circuit arrangement provided along the flow path of an input digital signal (i.e., along a signal processing sequence).
Conversely, in the case in which the error correcting encoder and decoder are provided inside the differential encoding/decoding circuits along the signal processing path, the adverse influence caused by the phase ambiguity in the reproduced carrier waves is not yet resolved at the input unit of the error correcting encoder. As a result, in such a case, it is required to employ such an error correcting code, namely a transparent error correcting code, even if the input signal is adversely influenced by the phase ambiguity in the reproduced carrier waves, e.g., there is bit inversion of the input signal, the error correction can be correctly performed with respect to the bit-inverted input signals.
As an error correcting code, there are a binary error correcting code and a nonbinary error correcting code. When a transparent binary error correcting encoder is employed inside differential encoding/decoding circuits, the transparency can be established by employing error correcting encoders/decoders in "n" pieces of a signal series. However, this system has a drawback in that when the multiple number of the QAM system is increased, the total number of the required error correcting encoders/decoders is also increased. In addition, there is a drawback in the binary error correcting code such that it is very difficult to produce a code whose coding rate is extremely high. When the decoding delay time of the error correcting code is, for instance, 63 symbols, even the resultant coding rate of the binary BCH (Bose-Chaudhuri-Hocquenghem) codes (63, 57) is 90.5%, by which a single error can be corrected, and thus the frequency band is expanded by approximately 10%. On the other hand, when the nonbinary error correcting code is employed, many difficulties may occur in realizing the above-described transparent conditions. Although it has been proposed that the signal point mapping of the QAM signal is the natural binary mapping and the Lee error correcting code is employed, since only such a case that errors occur in the signal points near the transmission signal points can be corrected based upon the Lee error correcting code, the error correcting effect cannot be expected in a communication channel or path which are subjected to a fading phenomenon. In addition, the coding rate of the Lee error correcting code is not always as good as other nonbinary codes.
As previously described, in the conventional QAM communication system employing the binary error correcting code, there are problems since the coding rate cannot be high so that the efficiency in the frequency utilization is lowered and also the total number of the required error correcting encoders/decoders to perform the differential encoding operation is necessarily increased. Furthermore, in accordance with the conventional QAM communication system employing the Lee error correcting code, there are drawbacks in that error correction can be executed limited only to the signal points having a small distance on signal point mapping.
The above-described problems of the conventional multi-level QAM communication system will now be described in detail.
That is, while the original data is reproduced from the received signal in the conventional multi-level QAM communication system, since the capture phase of the reproduced carrier wave has phase ambiguity such as 0, .pi./2, .pi. or 3.pi./2 radians, the two digital signal series to determine the quadrant of the phase plane are generally differential-encoded/decoded by employing the quadrant differential encoder/decoder.
On the other hand, there exist a natural binary mapping method, a Gray code mapping method and a quadrant symmetry mapping method as a signal point mapping method for mapping 2.sup.n pieces of signal points from the n bits of the digital signals.
As typical examples, FIG. 1 represents signal point mapping for a 16-QAM communication system employing the Gray mapping method, whereas FIG. 2 represents another signal point mapping for a 16-QAM communication system employing quadrant symmetry mapping. Further, FIG. 14 indicates signal point mapping employing natural binary mapping. As is apparent from FIG. 1, the respective signal points are symmetrically positioned with respect to the respective I and Q coordinate axes in Gray coded mapping. To the contrary, the signal points positioned in the respective quadrants are arranged in quadrant symmetry mapping in such a manner that these signal points are rotated with respect to those of the adjoining quadrants.
In these mapping methods shown in FIGS. 1, 2 and 14, the influences caused by the phase shifts of .pi./2, .pi., and 3.pi./2, which are given to the received signal series, are expressed in FIGS. 15A to 15C:
In general, it is known that the transmission capacity and frequency utilization efficiency in such a multi-level QAM communication system can be increased by increasing the signal points. However, the more the bit numbers are increased, the more the bit error rate is increased due to imperfections in the systems. It is desired that the error correction encoding/decoding operations be performed by slightly lowering the frequency utilization efficiency so as to improve the QAM communication quality.
Thus, as previously stated, in the case that the error correction encoder and decoder are provided outside the differential encoding/decoding circuits along the signal processing path, since the continuous bit errors are produced by the differential encoding operation, the error correcting capability of the error correction code must be emphasized or an interleaver must be employed.
However, when the error correcting capability of the error correction code is increased, the frequency utilization efficiency is deteriorated. When the interleaver is newly employed, not only the circuit scale of the entire system becomes large, but also the decoding delay time is increased. As a consequence, it is generally accepted to arrange such an error correction encoder/decoder inside the differential encoder/decoder.
It should be noted that when the error correcting encoder/decoder are arranged inside the differential encoder/decoder, error correction must be correctly performed even when the signal series are varied as represented in FIG. 14 due to the ambiguity of the capture phase in the reproduced carrier wave, and simultaneously, the phase ambiguity must be preserved even when the error correction encoding/decoding operations are carried out. It should be also noted that the error correction code which can satisfy such a condition is called as a transparent code with respect to phase rotation in an input signal.
Conventional circuit arrangements for the transparent codes with respect to the phase rotations in the input signals, have been proposed in Japanese KOKAI (Disclosure) patent application No. 63-219252, and "6GHZ 140MBPS DIGITAL RADIO REPEATER WITH 256QAM MODULATION" by Y. Yoshida et al., Proceedings of International Conference on Communications 1986, No. 46-7, pages 1482 to 1486.
In the multi-level QAM communication system as disclosed in the above-described Japanese KOKAI patent application No. 63-219252, there are various drawbacks. That is, since the error correction encoding/decoding operations are independently performed with respect to each of "n" pieces of a digital signal series which constitute the in-phase channel and also the channel orthogonal to the in-phase channel, "n" pieces of encoders and decoders are required. As a result, the scale of the entire apparatus becomes large.
On the other hand, in the multi-level QAM communication system as described in the above publication, i.e., ICC '86, No. 46-7, there is employed such an encoding/decoding method with employment of the Lee error correction code, for the respective signal series combinations between n/2 series combinations to constitute the in-phase channel and n/2 series combinations to constitute the orthogonal channel. However, this conventional communication system is limited to such a natural binary mapping method for mapping the n bits data to the signal points. Furthermore, there are many other limitations for the constituting methods of the error correction codes.